Refrigeration systems are often subject to changing operating conditions. In particular, the temperature of the system's coolant (water or outside air), and the temperature conditioning demand may vary. As the temperature conditioning demand varies, the system's compressor is usually controlled to operate between a fully to partially loaded condition to meet the demand. When the demand is very low, however, the compressor is typically cycled on and off. In some cases, excessive cycling may decrease the life of the compressor, and low load conditions may cause inadequate oil return to the compressor.
In many systems, excessive cycling is reduced by employing a hot gas bypass line. During periods of low load conditions, the hot gas bypass conveys relatively hot vaporous refrigerant directly from the condenser to the evaporator, bypassing the system's expansion device. The hot gas bypass flow lowers the effective capacity of the compressor which reduces the cycling. Although the efficiency of the system may be somewhat reduced, a hot gas bypass provides a very simple yet effective solution to the cycling problem.
Varying condenser coolant temperature may also cause problems with a refrigeration system. When the temperature of the condenser's coolant drops, the pressure differential across the expansion device decreases. If the coolant temperature becomes too low, the rate of condensation may exceed the mass flow rate leaving the condenser through the expansion device. Under such conditions, liquid refrigerant will start accumulating in the condenser. The liquid level will begin rising and flood much of the condenser, raising the condensing temperature and pressure. This continues until the condenser's effectiveness to condense refrigerant is reduced to a point of equilibrium where the rate of condensation equals the mass flow rate passing through the expansion device.
A partially flooded condenser deprives an otherwise flooded evaporator of its liquid refrigerant. A low level of liquid refrigerant in the evaporator reduces its heat transfer effectiveness, and so the overall efficiency of the system is reduced. Moreover, many systems rely on a high level of liquid refrigerant in the evaporator to assure proper oil return to the compressor. A high liquid level filling the majority of the evaporator with liquid, leaves a relatively small volume for refrigerant vapor. The relatively small volume forces refrigerant vapor to first pass at relatively high velocity across the surface of the liquid refrigerant in the evaporator before entering the compressor. The high velocity across the liquid refrigerant promotes the vapor's ability to entrain oil which is dissolved in the liquid refrigerant.
Problems brought about by low condenser coolant temperatures can be aggravated during periods of high temperature conditioning demand. During such periods, the compressor may need to operate fully loaded, i.e., compressor output is at its maximum mass flow rate. An increase in compressor output, in addition to the expansion device's lower pressure differential (caused by a low condenser temperature), leads to further accumulation of liquid refrigerant in the condenser. In other words, the mass flow entering the condenser (compressor output) is greater than the mass flow leaving the condenser by way of the expansion device (under the impetus of a reduced pressure differential).
It should be clear that several problems need to be addressed when designing refrigeration systems that may be operating under varying conditions. More specifically, the problems that need to be solved include excessive cycling during low load conditions, inadequate oil return during low load conditions, condenser flooding due to low condenser coolant temperatures, condenser flooding due to high load conditions, inadequate oil return due to insufficient evaporator flooding, and reduced heat transfer efficiency due to insufficient evaporator flooding and excessive condenser flooding. Although each of the above problems might be addressed separately, it is an object of the invention to provide a single apparatus comprising a simple bypass line having a valve that is controlled to provide the necessary flow to solve all the above problems.
Another object of the invention is to take full advantage of the heat transfer surface of both the condenser and evaporator during low head and high load operating conditions by decreasing condenser flooding and increasing evaporator flooding by conveying liquid refrigerant directly from the condenser to the evaporator, bypassing the expansion device.
Another object of the invention is to position a bypass line in a refrigeration system such that it provides a means to control the liquid level in both the condenser and the evaporator and also provides a hot gas bypass.
Yet another object is to provide a bypass line that is connected above the lowest point on the condenser so that the line is able to pass either liquid or gas, depending on the liquid level in the condenser.
A further object of the invention is to provide a bypass valve that is controlled in response to low load and high liquid level in the condenser.
A still further object is to determine a high liquid level in the condenser by using low head and high load conditions as indicators.
Yet another object is to provide proper oil return to the compressor at low head/high load conditions by maintaining a proper level of liquid refrigerant in the evaporator so that before vaporous refrigerant enters the compressor, the vapor first passes through the evaporator at sufficiently high velocity to entrain oil at the surface of the liquid refrigerant.
Another object is to enhance oil return at low load conditions by strategically locating the outlet of the hot gas bypass below the liquid level in the evaporator.
Another object of the invention is to provide a variable capacity refrigeration system with both a variable capacity compressor and a hot gas bypass.
And another object is to provide a method of maintaining a proper level of liquid refrigerant in both the condenser and evaporator of a variable capacity refrigeration system having a fixed orifice for an expansion device.
These and other objects of the invention will be apparent from the attached drawings and the desription of the preferred embodiment that follows below.